With the current development of information science and technology, the emergence of new technologies, such as automatic driving, remote medical care, and cloud computing, has dramatically increased the demand for information processing speed and capacity. It can be used to handle computing tasks that are difficult to solve by traditional computers, such as cryptography, molecular simulation, and big data processing, and has high speed and low power consumption. Those characteristics can meet the rapidly growing computational needs. The development of large-scale applications of optical quantum technology requires densely integrating nanophotonic devices on photonic chips. Therefore, information interaction and computation based on photonic chips is a fulcrum technology for the entire information field in the future and has a far-reaching market and strategic significance in information technology, national defense and security, energy, health care and other fields.

The physical nature of photons determines that optical signals cannot be stored directly in a medium like electronic signals. Data cache is a key module in optical communications and computing networks. The existing all-optical data cache is implemented through various adjustable optical delay lines. In integrated chips, optical delay lines can be easily combined with other functional devices to provide more powerful optical and microwave processing capabilities than single devices, improving their performance in terms of stability, tuning speed and power consumption. Therefore, designing tunable optical delay lines has been a research focus in recent years.

Optical delay lines can be realized through a variety of methods, including single or cascaded microring resonators, waveguide gratings, photonic crystals, multipath reconfigurable delay networks, loops and other structures. The key performance indicator of the optical delay line is the delay time , where is the group refractive index, c is the speed of light, and L is the optical path length of an optical delay line. It is difficult to achieve a high group refractive index with traditional optical delay lines, which need a long optical path to achieve the required delay time. Therefore, large device size is necessary, which is also accompanied by problems such as high transmission loss, limited tuning range, and inability to control photon spin.

The slow light effect can slow down the group velocity and increase the group refractive index, providing the possibility to solve the above bottleneck issues. However, the slow-light effect is generally accompanied by strong scattering losses. Meanwhile, there are still challenges to achieve ultra-strong slow-light effects to achieve ultra-high group refractive index. Therefore, achieving ultra-strong slow-light effects and reducing scattering losses are core issues for realizing miniaturized and high-performance integrated optical delay lines.

The research team of Professor Hongming Fei at the Taiyuan University of Technology collaborated with Professor Liantuan Xiao to propose a tunable optical delay line in the visible light band based on a two-dimensional hexagonal boron nitride valley photonic crystal structure. The tuning function of slow light wavelength and delay time is achieved by controlling the optical properties of the liquid crystal material using an external voltage. This optical delay line has the advantages of compact structure, high transmittance, and tunability. It provides an effective solution for designing ultra-compact, high-performance integrated optical delay lines and can be used in optical communications and quantum computing. The related work was published in Chinese Optics Letters, vol. 22, no. 5, 2024 (Hongming Fei, Min Wu, Han Lin, Yibiao Yang, and Liantuan Xiao. A tunable hexagonal boron nitride topological optical delay line in the visible region, Chinese Optics Letters, 2024, 22(5), 053602), and was selected as the cover of the current issue.

Fig. 1 (a) Schematic of the optical delay line structure of the valley photonic crystal; (b) Dispersion curves of the edge-state; (c) Plots of group refractive index and group velocity versus frequency and wavelength, respectively; (d) Plots of the group refractive index and group velocity versus the wave vector

First, based on the hexagonal boron nitride material that can achieve high transmission of visible light, the energy valley photonic crystal structure is designed based on the scaling effect of the photonic crystal to realize the optical delay line in the visible wavelength range (shown in Fig. 1(a)). And the high group refractive index (~629) is realized by the unique slow light effect of the edge state of the valley photonic crystal (shown in Fig. 1(b)-(d)), which effectively shortens the optical path and realizes the ultra-compact optical delay line with long delay time. On the other hand, the unique spin-valley locking effect of the valley photonic crystal can effectively reduce the scattering loss to achieve high transmittance. The tuning of slow-light wavelength is further realized by adjusting the effective refractive index (refractive index and radius dielectric columns) of the edge structure of the valley photonic crystal. In addition, the tuning of slow-light wavelength and optical delay time is achieved by injecting a liquid crystal material into the edge-state waveguide structure and tuning the refractive index of the liquid crystal material under an external voltage. The design will open up new possibilities for tunable topological photonic devices in the visible region. The optical delay line can be widely applied in optical communications, optical quantum computing, optical signal synchronization and caching, beamforming and modulation, optical coherence tomography, time-division multiplexing, and building integrated optical gyroscopes.

In recent years, the demand for optical computing technology has increased rapidly due to the following reasons: (1) With the gradual invalidation of Moore's Law and the continuous increase in the power consumption and speed requirements of computation systems in the era of big data, optical computing technology has attracted more and more attention due to the high-speed and low power consumption; (2) The parallel computing characteristics of optical computing technology, as well as the development of algorithms and hardware architectures such as optical neural networks, have provided a new platform for artificial intelligence technologies such as image recognition, speech recognition, and virtual reality. As a result, the demand for photonic chips is increasing day by day. Among them, topological photonic crystals have become a research focus because of their unique defect immune unidirectional transmission property. This work chose the two-dimensional hexagonal boron nitride material with unique properties to design a topological photonic crystal device. On the one hand, it broadens the operating band of the topological photonic crystal device. On the other hand, because the boron nitride material can realize a high-performance quantum light source, it is expected to be directly combined with the topological photonic crystal structure to realize integrated photonic chips, contributing to the development of photonic chips.